The present invention relates generally to a system and method to facilitate power saving for a wireless station.
The Institute of Electrical and Electronic Engineers (IEEE) 802.11e draft standard defines two access methods for Quality of Service (QoS) streams, EDCA (Enhanced Distributed. Channel Access) and HCCA (Hybrid Controlled Channel Access).
EDCA channel access is contention based. EDCA channel access parameters are prioritized so that channel access delay is statistically decreased for high priority streams. On lightly loaded channels, EDCA minimizes channel latency; however, EDCA is susceptible to congestion as the traffic load increases, especially in networks with hidden nodes. Herein, a first wireless station is “hidden” from a second wireless station if the second wireless station cannot detect transmissions by the first wireless stations; however, simultaneous transmissions by the wireless stations will collide at a receiver (e.g. a common parent access point).
An AP (access point) may use the highly-prioritized “contention-controlled” HCCA access method to gain control of the channel. HCCA utilizes polling by a parent AP to grant channel access to child stations. HCCA is much less susceptible to collisions caused by hidden nodes since an AP is never hidden from its child stations. HCCA provides more deterministic access for QoS streams; however, HCCA adds latency because frames are only sent at scheduled polling times. An AP (access point) initiates a bi-direcitonal HCCA burst with a station by sending a data+poll frame to the station. An AP can only send a data+poll frame to the power-save HCCA station at the start of each scheduled wakeup time. The power-save station sends uplink frames in a scheduled wakeup period in response to a downlink poll frame.
An access method developed for power-save VoIP (Voice over Internet Protocol) stations is commonly referred to as “reverse polling.” Reverse polling is based on EDCA channel access and the 802.11e “Unscheduled Advanced Power-Save Delivery” (U-APSD) method. With reverse polling, frame exchanges between a power-save wireless station and a parent AP are always initiated by the wireless station. A U-APSD power-save VoIP station wakes up whenever it has a voice sample ready for transmission and sends an uplink voice frame to the AP. The uplink frame establishes an unscheduled wakeup period for the station. The AP then delivers downlink frames to the client during the unscheduled wakeup period. Reverse polling minimizes latency for the uplink stream, but is susceptible to congestion since it is EDCA based. If silence suppression is enabled, reverse polling also adds to the channel load because a U-APSD VoIP station generates “null” uplink frames to establish unscheduled wakeup periods during periods of uplink silence suppression.
In the IEEE 802.11e QoS standard, both HCCA and EDCA access cannot be used for the same QoS stream.